This section is intended to introduce the reader to various aspects of art, which may be associated with exemplary embodiments of the present invention, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with information to facilitate a better understanding of particular techniques of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not necessarily as admissions of prior art.
The production of hydrocarbons, such as oil and gas, has been performed for numerous years. To produce these hydrocarbons, a wellbore is typically drilled in intervals with different casing strings installed to reach a subsurface formation. The casing strings are installed in the wellbore to prevent collapse of the wellbore walls, to prevent undesired outflow of drilling fluid into the formation, and/or to prevent the inflow of fluid from the formation into the wellbore. Typically, the process of installing casing strings involves tripping, running casing, and cementing the casing strings. Because the casing strings in the different intervals pass through already installed casing strings, the lower intervals of the casing strings typically have smaller diameters. In this manner, the casing strings are formed in a nested configuration that continue to decrease in diameter in each of the subsequent intervals.
In addition to the casing strings, a drilling mud is circulated within the wellbore to remove cuttings from the well. The weight or density of the drilling mud is typically maintained between the pore pressure gradient (PPG) and the fracture pressure gradient (FG) for drilling operations. However, the PPG and FG increase along with the true vertical depth (TVD) of the well, which present problems for maintaining the drilling mud weight. If the weight of the drilling mud is below the PPG, the well may take a kick. A kick is an influx of formation fluid into the wellbore, which has to be controlled for drilling operations to resume. Also, if the weight of the drilling mud is above the FG, the drilling mud may leak off into the formation. These lost returns result in large volumes of drilling mud loss, which has to be replaced for the drilling operations to resume. Accordingly, the casing strings are utilized to assist in maintaining the weight of the drilling mud within the PPG and FG to continue drilling operations to greater depths.
With subsurface formations being located at greater depths, the cost and time associated with the forming the wellbore increases. For instance, with the nested configuration, the initial casing strings have to be sufficiently large to provide a wellbore diameter of a specific size for the tools and other devices near the subsurface formations. As a result the diameter of the initial casing strings is relatively large to provide a final useable wellbore diameter. The large diameter increases the costs of the drilling operations because of the cost associated with the increased size of the casing string, increased volume of cuttings that have to be managed, and increased volume of cement and drilling mud utilized to form the wellbore. As such, the cost of typically drilling operations results in some subsurface formations being economically unfeasible.
To reduce the diameter of casing strings, various processes are utilized. For example, drilling operations may utilize variable density drilling mud to maintain the drilling mud within the PPG and FG. As noted in Intl. Patent Application Publication No. WO 2006/007347 to Polizzotti et al., compressible objects may include compressible or collapsible hollow objects of various shapes or structures. These compressible objects, which are selected to achieve a favorable compression in response to pressure and/or temperature changes. These compressible objects may be recirculated as part of the variable density drilling mud to provide volume changes that reduce the number of intermediate casing string intervals in the wellbore.
However, the use of compressible objects in the variable density drilling mud can be challenging. For instance, the compressible objects have to be fabricated to provide a certain amount of compression and to be resilient. Further, the compressible objects have to be designed to compress at certain pressures to provide the volume changes in specific intervals within the wellbore. In addition, the drilling fluid, which is combined with the compressible objects, may be selected and include certain additives to interact with the compressible objects to enhance the variable density drilling mud. As such, there is a need for a method for selecting and fabricating compressible objects for use with drilling fluids to form the variable density drilling mud.
Other related material may be found in at least U.S. Pat. Nos. 3,174,561;  3,231,030; 4,099,583; 5,881,826; 5,910,467; 6,156,708; 6,422,326; 6,497,289; 6,530,437; 6,588,501; 7,108,066; U.S. Patent Application Publication No. 2005/0113262; U.S. Patent Application Publication No. 2005/0284641; and Intl. Patent Application Publication No. WO 2006/007347.